Because almost all drug trials for Alzheimer's disease (AD) have failed, developing non-pharmacological interventions with strong potential to prevent or delay the onset of AD in high-risk populations (e.g., those with mild cognitive impairment [MCI]) is critically important. Aerobic exercise and cognitive training are 2 promising interventions for preventing AD. Aerobic exercise increases aerobic fitness, which in turn improves brain structure and function, while cognitive training improves selective neural function intensively. Hence, combined Aerobic exercise and Cognitive Training (ACT) may very well have an additive or synergistic effect on cognition by complementary strengthening of different neural functions. Few studies have tested ACT's effects, and those studies have reported discrepant findings, largely due to varying ACT programs. The purpose of this single-blinded, 22 factorial Phase II randomized controlled trial (RCT) is to test the efficacy and additive/synergistic effects of a 6-month combined cycling and speed of processing (SOP) training intervention on cognition and relevant mechanisms (aerobic fitness, AD signature cortical thickness, and functional connectivity in the default mode network [DMN]) in older adults with amnestic MCI (aMCI). Our preliminary studies have shown that enhanced aerobic fitness is associated with better cognition and resting-state functional connectivity in the DMN in AD, and ~20 hours of SOP training improves executive function and maintains functional connectivity in the DMN in aMCI. This RCT will randomize 128 participants equally to 4 arms: ACT, cycling only, SOP training only, or attention control for 6 months, and then follow them for another 12 months. Cognition and aerobic fitness will be assessed at baseline, 3, 6, 12, and 18 months; AD signature cortical thickness and functional connectivity in the DMN at baseline, 6, 12, and 18 months; AD conversion at 3, 6, 12, and 18 months. The specific aims are to: Aim I. Determine the efficacy and additive/synergistic effects of ACT on cognition over 6 months. H1: ACT will have the greatest effects on executive function and episodic memory compared with other groups. Aim II. Examine the underlying mechanisms of ACT over 6 months. H2a: ACT will have the greatest effects on AD signature cortical thickness, functional connectivity in the DMN, and aerobic fitness compared with other groups. H2b: Changes in the mechanistic measures are related to cognitive changes. H2c: Changes in AD signature cortical thickness and DMN mediate aerobic fitness' effects on cognition. Aim III (exploratory). Calculate the long-term effect sizes of ACT on cognition and clinical and pathological AD conversion to inform future Phase III RCTs. Analysis will use intention-to-treat and linear mixed-effect modeling. This trial will be the first to test the synergistic effects on cognition and mechanisms (relevant to AD-associated neurodegeneration) of a uniquely conceptualized and rigorously designed ACT in older adults with aMCI. It will advance AD prevention research by providing precise effect-size estimates of the ACT intervention. Our long-term goal is to delay AD onset and slow AD progression.